Components located in the high temperature section of gas turbine engines are typically formed of superalloys, which includes nickel-base superalloys, iron-base superalloys, cobalt-base superalloys and combinations thereof. High temperature sections of the gas turbine engine include the combustor section and the turbine section. In some types of turbine engines, the high temperature section may include the exhaust section. The different hot sections of the engine may experience different conditions requiring the materials comprising the components in the different sections to have different properties. In fact, different components in the same sections may experience different conditions requiring different materials in the different sections.
Turbine buckets or airfoils in the turbine section of the engine are attached to turbine wheels and rotate at very high speeds in the hot exhaust gases of combustion expelled by the turbine section of the engine. These buckets or airfoils must simultaneously be oxidation-resistant and corrosion-resistant, maintaining their microstructure at elevated temperatures of use while maintaining mechanical properties such as creep resistance/stress rupture, strength and ductility. Because these turbine buckets have complex shapes, in order to reduce costs, they should be castable to reduce processing time to work the material as well as machining time to achieve the complex shapes.
Nickel-base superalloys have typically been used to produce components for use in the hot sections of the engine since they can provide the desired properties that satisfy the demanding conditions of the turbine section environment. These nickel-base superalloys have high temperature capabilities, while achieving strength from precipitation strengthening mechanisms which include the development of gamma prime precipitates. The nickel-base superalloys in their cast form are utilized for buckets and currently are made from nickel-base superalloys such as René N4, René N5, which form high volume fractions of gamma prime precipitates when heat treated appropriately, and GTD®-111, Rene 80 and In 738, which form somewhat lower volume fractions of gamma prime precipitates when heat treated appropriately. GTD® is a trademark of General Electric Company, Fairfield, Conn. Other nickel base superalloys forming even lower volume fractions of gamma prime, such as GTD® 222 and IN 939 are used in lower temperature applications, such as nozzle or exhaust applications.
High weight percentages of nickel add to the cost of nickel-base superalloys because nickel is an expensive material. In addition, nickel is a strategic alloy, being used in many critical industries around the globe. Even though it is a strategic resource, primary sources of nickel are Australia, Canada, New Caledonia and Russia. Currently, there is only one working nickel mine in the United States. So, finding an effective low-cost substitute for nickel is beneficial both from a cost perspective and from a strategic perspective.
What is needed is a low cost substitute for nickel in superalloys, such as nickel-base superalloys. More specifically. for turbine applications, what is needed is a readily available low cost substitute for nickel-base superalloys that can be used without affecting the high temperature mechanical properties of the alloy included such properties as creep/stress rupture, tensile properties as well oxidation resistance, corrosion resistance and castability.